Donating under Gift Aid means that the charity you are donating to will be able to recover 28p for every £1 you donate from the Taxman. This is available until 5th April 2011.
For your charity to recover this cash you must give a Gift Aid declaration. Where you make a donation over the phone instead of using a website, your charity will write to you to confirm this Gift Aid declaration.
Another requirement is that you must be a taxpayer who has paid tax that is at least the amount that you charity will claim under Gift Aid. If you are not a taxpayer you cannot make a Gift Aid declaration.
If you are a higher rate tax payer you can also claim the difference the 40% high rate of tax and 20% basic rate of tax on your personal tax return. You need to keep your accountant informed of donations you have made under gift aid so he/she can claim the difference.